According to the general name of power electronic devices is semiconductor power devices, it is the basis of power electronic equipment, is engaged in power electronic device design, research and development, production, marketing and application personnel and power supply technicians should be familiar with. This journal started to give lectures on the topic of "Power Electronics Device Knowledge" in April this year to meet the readers' needs to increase knowledge and make good use of these devices.
Engineers of manufacturers and users are welcome to write articles, and hope to put forward valuable comments.
Lectures on Power Electronic Devices (7) Insulated Gate Bipolar Transistor (IGBT) (-) Qiao Enming Xue Yujun Liu Min (Contributed) Zhang Naiguo (adapted) of the editorial department of the journal (InsulatedGateBipolarTransistor, referred to as IGBT) A bipolar self-turn-off device with voltage control composed of MOSFET and GTR. The saturation voltage of GTR decreases, the current carrying density is large, but the driving current is large, and the driving circuit is complex; the power of the MOSFET is small, the driving circuit is simple, the switching speed is fast, but the turn-on voltage drop is large, and the current carrying density is small. IGBT combines the advantages of GTR and power MOSFET. It has the characteristics of high input impedance, fast switching speed, small driving power, low saturation voltage, simple control circuit, high voltage resistance, high current withstand, etc. It is widely used in various power conversions. application. Due to the optimization of the design and the application of the technology of large-capacity memory in recent years, its characteristics have been greatly improved, and the scope of application has exceeded the GTR and power MOSFET in the past. Since it was put on the market in 1986, IGBT has rapidly expanded its application In the field, it has replaced the market of GTR and some power MOSFETs and become the leading device of medium and high-power power electronic equipment. It is not only used in power systems, but also widely used in general industry, transportation, communication systems, computer systems, and new energy. The system is also used in household appliances such as lighting and air conditioning. At present, IGBT products have been serialized. The highest withstand voltage is 6500V and the current is 1200A, and efforts are continuing to improve voltage, current capacity and switching frequency. It has become one of the most widely used power electronic devices.
1 The basic structure of IGBT 1.1 The basic structure of IGBT The insulated gate bipolar transistor is essentially a field effect transistor, similar in structure to a power MOSFET, except that an additional P + layer is added between the drain and drain regions of the original power MOSFET . Shown is a cross-sectional view of the structure of an N-channel enhanced insulated gate bipolar transistor. The N + region is called the source region, and the electrode attached to it is called the "emitter" (equivalent to the source electronic component in the power MOSFET Knowledge lectures ==> pole). The control area of ​​the device is the gate area, and the electrode attached to it is called the "gate". The N- layer is the drift region, and the N + layer is the buffer zone (this is not necessary in the IGBT, the specific content will be described in detail later). In the structure of the IGBT, the gate and source are similar to the power MOSFET. The difference between the structure of the IGBT and the power MOSFET is that the IGBT adds a P + layer on the N + layer of the N-channel power MOSFET, forming a This drain is called "collector" in IGBT.
In the IGBT, the P region connected to the emitter, the drift region N- region, the buffer zone N + region, and the P + region form a PNP-type transistor (see illustration). The P + type injection layer connected to the collector is a unique functional area of ​​the IGBT. It acts as a collector, injects holes into the N- area of ​​the drift area, conducts conductance modulation on the N- area of ​​the drift area, and makes the drift area of ​​the IGBT in the on state The N-region maintains a high carrier concentration to reduce the on-state voltage of the device.
The turn-on and turn-off of the IGBT are controlled by the gate voltage. When a positive voltage is applied to the gate, a conductive channel for electron carriers is formed in the P region below the gate. The electron carriers are injected into the N- drift region from the N + region of the emitter through the conductive channel, which is the internal IGBT. The PNP-type transistor supplies the base current, so that the PNP-type transistor turns on, and also turns on the IGBT. At this time, in order to maintain the electrical balance of the N- drift region, the P + region injects hole carriers into the N- drift region, and maintains the drift region N- region with a higher carrier concentration, that is, the N- drift region Conductance modulation is performed to reduce the on-resistance of the drift region, so that the high withstand voltage IGBT with a long drift region also has a low on-state voltage drop. If a negative voltage is applied to the gate, the channel in the MOSFET disappears, the base current of the PNP transistor is cut off, and the IGBT is turned off.
In fact, in the IGBT structure, in addition to the internal PNP type transistor analyzed above, there is also an NPN type transistor, which is composed of an N + region, a P body region, and a drift region N- region connected to the emitter. In order to prevent the thyristor effect of the combination of the PNP type and NPN type transistors, the NPN type transistor is designed to be ineffective as much as possible during design. For example, the aluminum electrode of the emitter and base of the NPN type transistor is short-circuited. Therefore, the basic operation of the IGBT has nothing to do with the NPN transistor, which can be equivalent to a Darlington tube with an N-channel MOSFET as the input stage and a PNP transistor as the output stage. IGBT is equivalent to a thick base PNP transistor driven by MOSFET, as shown. IGBT combines the advantages of the fast switching characteristics of power MOSFETs and the reduction of the on-voltage of bipolar transistors. IGBT is a composite device with bipolar transistors as the leading component and MOSFET as the driving component.
1.2 The equivalent circuit and graphic symbol of IGBT, the graphic symbol of IGBT is shown, where the direction of the arrow indicates the direction of current flowing when the IGBT is turned on.
The blocking principle of IGBT IGBT and MOSFET are the same, and the device can be turned on and off by controlling the driving voltage between the gate and the emitter. The principle of IGBT forward blocking is similar to MOSFET. When the gate voltage Uge is lower than the threshold voltage Ut, no N-type conductive channel is formed in the P body region under the gate of the IGBT, and the device is in a blocking state. The forward voltage between the collector and the emitter biases the PN junction 2 reversely. The voltage between the collector and the emitter is almost entirely borne by the PN junction 2. At this time, only a very small leakage current passes through the drift region. The electrode flows to the emitter.
IGBT conduction principle When the gate voltage applied to the IGBT is higher than the threshold voltage Ut, like the MOSFET, the P body region below the gate of the IGBT will form a conductive channel, connecting the N-drift region with the IGBT The N + regions below the emitter are connected. As shown, a large number of electrons are injected into the N- drift region from the emitter through the conductive channel and become the base current of the internal PNP transistor. Due to the positive bias of the sense junction, a large number of holes are injected into the N- drift from the injection region P + Area. The holes injected into the N-drift region flow through the drift region through drift and diffusion, and finally reach the P body region. After the hole enters the P body region, it attracts a large amount of electrons from the metal in contact with the emitter. These electrons are injected into the P body region and quickly recombine with the hole to form the on-current of the device. The IGBT is in the on state.
Shown is the structure of the equivalent MOSFET inside the IGBT and the bipolar transistor GTR. When the IGBT is turned on, its current is turned on through the internal equivalent MOSFET and GTR. Shown is the equivalent circuit of the IGBT, which is formed by connecting a MOSFET and a bipolar transistor Darlington. The IGBT turn-on voltage drop is the voltage drop across the body resistance; Rchannel is the equivalent on-resistance of the P body region. Because of the conductance modulation effect in the IGBT, Um is much smaller than the turn-on voltage drop of the power MOSFET under the same operating conditions, so that the turn-on voltage drop of the entire IGBT will be smaller than the turn-on voltage drop of the MOSFET.
2.3 IGBT latching effect In IGBT, the internal PNP bipolar transistor and parasitic NPN bipolar transistor constitute a thyristor. As shown, there is a latching effect when the thyristor turns on. The effect of IGBT can be divided into static effect and dynamic effect.
The static pinning effect occurs in the on-state IGBT. There are two transistors inside the IGBT, namely a PNP transistor and an NPN transistor, and an equivalent body resistance Rk is connected in parallel between the base and emitter of the NPN transistor. When the IGBT is turned on, current flows through the body The regional resistance Rb also produces a certain voltage drop. For the base of the NPN transistor, it is equivalent to adding a forward bias voltage. Within the specified collector current range, this forward bias voltage is not large enough, so the NPN transistor will not turn on. However, when the collector current increases to a certain value, this forward bias voltage will turn on the NPN type transistor, and mutually stimulate with the PNP type transistor, forming a positive current inside the two transistors similar to when the thyristor is turned on The feedback phenomenon causes the collector current to rise rapidly and reach saturation. If the gate control signal of the IGBT is removed at this time, the IGBT will still be in the on state, which means that the gate of the IGBT will lose control at this time. This phenomenon This is called the "static anchor effect".
The IGBT also produces a latching effect during the turn-off process, called the dynamic latching effect. When the IGBT is turned off, the MOSFET function unit inside the IGBT is turned off very quickly. The reverse voltage is quickly established at the 2 junction. The voltage change at the 2 junction causes the displacement current CdUDs / dt, which will be on the body resistance Rbr Generates a voltage that forward-biases the internal parasitic NPN transistor. Therefore, the faster the IGBT is turned off, the faster the voltage changes at the 2 junction, and the resulting displacement current is also greater. When the displacement current exceeds a certain threshold, the NPN transistor will be forward-biased, forming a similar The positive current feedback phenomenon in the conduction process of the thyristor produces a "dynamic anchoring effect". The dynamic latch-up effect is mainly determined by the rate of voltage change, and is also affected by factors such as collector current Icm and junction temperature. The collector current allowed by the dynamic pinning effect is smaller than that during static pinning, so the critical collector current Icm specified by the manufacturer (vendor) is determined by the maximum collector current allowed without dynamic pinning effect of. Therefore, when using IGBT, the collector current of the IGBT must be limited to be less than the maximum collector current Icm specified by the manufacturer (commercial). Increasing the gate drive resistance will extend the IGBT off time and help reduce the voltage The rate of change limits the dynamic effect of IGBT.
Knowledge Lecture> => 3 IGBT characteristics and parameters 3.1 Static characteristics of IGBT The static characteristics of IGBT refers to the relationship curve between the IGBT on-state current and the collector-emitter voltage Uce when the gate drive voltage Uge is used as a parameter. Under a certain collector-emitter voltage Uce, the collector current is controlled by the gate drive voltage Uge. The higher the Uge, the larger the IC. The volt-ampere characteristics of IGBT are usually divided into four parts: saturation region, linear amplification region, forward blocking region and forward breakdown region, as shown. When the IGBT is turned on, the IGBT should be operated in the saturation region; when the IGBT is off, the applied voltage is borne by the 2 junction. It should be ensured that the IGBT is in the forward blocking region. At this time, the maximum collector-emitter voltage should not exceed Breakdown voltage Ubr. Is a schematic diagram of IGBT transfer characteristics. The transfer characteristic represents the relationship between the IGBT collector current / (and the gate drive voltage UGE. The transfer characteristic of the IGBT is similar to that of the MOSFET.
When the gate drive voltage is less than the threshold voltage Ut, the IGBT is in the off state. In most of the collector current range after the IGBT is turned on, when the gate drive voltage is higher than the threshold voltage Ut, the collector current of the IGBT increases as the gate drive voltage increases. The maximum gate drive voltage is limited by the maximum drain current, and its optimal value is generally about 15V.
In the large current region, under the condition of the same gate drive voltage, the collector current decreases as the temperature rises, showing a negative temperature coefficient characteristic. It is also mainly affected by the internal conductivity modulation effect of the IGBT.
3.3 Switching characteristics of IGBT The switching characteristics of IGBT are divided into two parts: one is the switching speed, the main index is each part of the time in the switching process; the other is the loss in the switching process. It is a commonly used IGBT switch characteristic test circuit. This circuit is a chopper circuit, where V (IG-BT) is used as a switch, and the diode anti-parallel to VE (IGBT) is used as a freewheeling diode for inductive load. Here, the gate drive signal of VE is constant at -15V, so VE is always It is in shutdown state. By controlling the gate drive signal of the switch tube TVi (IGBT), the TVi is turned on and off, and the current and voltage waveforms of the IGBT during the switching process are tested. 0 shows the waveform of the IGBT switching process, from top to bottom are the IGBT gate drive voltage Uge, the IGBT collector-emitter voltage Uce and the IGBT collector current 1C. In the figure, the various stages of the IGBT switching process Time is defined. In the entire switching process, it can be roughly divided into eight time periods. Here, the time during the switching process is defined as follows: (1) tn: turn-on time, the time from the effective turn-on signal generation to the complete conduction of IG-BT.
td): Turn-on delay time, the time required from the generation of the effective turn-on signal to the rise of the IGBT collector current Ic to the amplitude of 10% at turn-on.
tn: the current rise time, the time from when the current rises from 10% to 90%.
The turn-off time is the time required from the generation of an effective turn-off signal to the complete turn-off of the IG-BT (current is 10% of the turn-on time).
tdW: Turn-off delay time, the time from the moment when the drive voltage drops to its 90% amplitude to the time when the IGBT collector current drops to 90% of its amplitude.
f: Current fall time, the time for the IGBT current to fall from 90% to 10% when it is turned on.
t: reverse recovery time of built-in diode.
3.4 The switching process of IGBT In the process of IGBT opening, most of the time it operates as MOS-FET, only in the late stage of the collector-emitter voltage Uce decline, the PNP transistor enters the saturation region from the amplification region, adding a delay time. Therefore, most of its turn-on process is similar to that of MOSFET. Figure 1 shows the IGBT gate drive voltage, collector current, and collector-emitter voltage waveform during the open pass.
After the drive signal is removed, the IGBT's turn-off voltage and current waveforms are shown in 2. In the figure, from top to bottom are the gate drive voltage Uge, collector current Ic, and collector-emitter voltage Uce waveform. It can be clearly seen from the figure that before the collector current begins to decrease, the collector-emitter voltage Uce has a rising process to a steady state. At the beginning of the turn-off process, there is a turn-off delay time td (. ï¿¡, and then due to the presence of the MOSFET component in the IGBT, a collector-emitter voltage rise time t is generated
Shenzhen Esun Herb Co.,Ltd. , https://www.szyoutai-tech.com